There are known devices for detecting excessive current draw in an electrical load. Typically, such detector devices sense the amount of current flowing through the electrical load. When the sensed current exceeds a predetermined value, an output signal indicative thereof is provided by the detector. The detector output signal is typically used to control a switching device. When the detector output signal is indicative of excessive current draw, the switching device disconnects the electrical load from the source of electrical energy.
One particular type of excessive current detecting device is a short circuit detector. Such detectors typically include means for sensing the amount of electrical current drawn by an electrical load. A predetermined test value is established which is slightly greater than the maximum expected value of current draw by the electrical load under normal, steady-state operating conditions. If the actual current draw by the load exceeds the predetermined test value, it is assumed that the electrical load is short circuited. The detector, upon detecting a short circuit, controls a switching device to disconnect the electrical load from the source of electrical energy.
Some electrical loads, such as lamps or electric motors, for example, initially draw a large amount current when a source of electrical energy is first applied thereto. Under normal operating conditions, the large, initial current drawn by the load subsequently decays to a steady-state, operating value. The large, initial current draw is known in the art as "in-rush current." A short circuit detector, upon sensing an in-rush current that exceeds its predetermined test value, would disconnect the electrical load from the energy source when, in fact, there was no true electrical short circuit in the electrical load.
To prevent a false indication of an electrical short circuit in a load having a high in-rush current, some electrical short circuit detectors have included time-delay circuits. Such time-delay circuits disable the short circuit detector for a predetermined period of time after initial application of electrical energy to the electrical load. The time period is sufficiently long to insure that, if the electrical load is operating normally, the current draw by the load will decay to its steady-state value before the period expires.
An example of an electrical short circuit detector having a time-delay circuit is disclosed in U.S. Pat. No. 3,465,206 to Harker et al. The Harker et al. '206 patent discloses a time-delay circuit breaker having a short circuit override. An electrical load is connected to a source of electrical energy through relays which are controlled by a relay drive circuit. The relay drive circuit is operatively connected to two silicon-controlled recitfiers (SCR's) which are connected in series. A current monitoring circuit is electrically connected to both SCR's and to lines connecting the relays to the load. The SCR's are arranged to trip when their gate voltages reach a predetermined level.
During an excess, steady-state current draw, the SCR's are tripped. When the SCR's are tripped, the relay drive circuit deactivates causing the relays to open thereby disconnecting the electrical load from the source of electrical energy. When the source of electrical energy is initially applied to the load, a time-delay circuit sufficiently delays actuation of the SCR's so as to prevent a false indication of an electrical short circuit. If a true electrical short circuit exists at the time of initial application of electrical energy, a signal is generated which bypasses the time-delay circuit and instantaneously trips the SCR's.
Another example of an electrical short circuit detector is disclosed in U.S. Pat. No. 4,544,981 to Hakoopian. The Hakoopian '981 patent discloses a short circuit protection/controller circuit for drivers and amplifiers. Current sensing resistors sense the current flowing through the positive and negative power supply input lines to the amplifier. Overload or short circuit conditions are detected by using comparators to compare the voltage drop across each current sensing resistor against a reference voltage. The amplifier is disconnected from the load when either current sensing resistor provides an electrical signal indicative of overload or short circuit conditions. A capacitor is used to suppress transient or noise currents which exceed the overload or short circuit current limit value.